1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an image processing apparatus.
2. Description of the Related Art
In a serial type ink jet printing apparatus, a printing scan in which a printing head is moved while causing the printing head to eject ink in accordance with image data and a conveyance operation to convey a printing medium in a direction intersecting the direction of the printing scan are repeated alternately. At this time, there is a case where a joint streak occurs at the boundary part between the regions in which printing is performed by printing scans performed twice successively in the printing medium.
For example, Japanese Patent Laid-Open No. H08-25693 (1996) has disclosed the method for causing printing regions in which printing scans are performed twice successively to overlap each other to a certain extent in the conveyance direction and adjusting the number of dots that are printed in the overlapped region by using a mask pattern. At this time, by using a gradation mask pattern by which the printing ratio at end parts of the printing head is reduced gradually, it is possible to suppress unevenness in density in each overlapped region.
Further, Japanese Patent Laid-Open No. 2002-36524 has disclosed the method for counting the number of dots that are printed in the vicinity of the boundary part and adjusting the thinning ratio at the boundary part in accordance with the number of counted dots without providing an overlapped region. Conspicuity of the joint streak depends on the gradations, i.e., the number of dots that are printed, but by adopting Japanese Patent Laid-Open No. 2002-36524, it is possible to appropriately adjust the number of dots at the boundary part in accordance with the gradations, and therefore, it is made possible to make less conspicuous the joint streak regardless of the density. On the other hand, Japanese Patent Laid-Open No. 2008-922 has disclosed a method for adding dots at the joint part region by focusing on, in particular, a white streak.
However, in the case where Japanese Patent Laid-Open No. H08-25693 (1996) is adopted, the number of times of printing scan required to print an image is different between the overlapped region and the other region. Specifically, in the case of the one-pass printing, in the overlapped region, ink is given during the printing scans performed twice, but in the other region, ink is given during the printing scan performed once. As a result of that, for example, in the case where printing is performed on glossy paper by using, for example, a pigment ink, there is produced a difference in the degree of irregularities of the image surface, i.e., smoothness between the overlapped region and the other region, and the difference may be erroneously recognized as glossiness unevenness. Further, in the case where mixed-color printing is performed by using inks in two or more colors, there is produced a difference in the color tone and saturation between the overlapped region and the other region and the difference may be erroneously recognized as color unevenness.
According to the intensive examination by the inventors etc., of the present invention, even in the case where Japanese Patent Laid-Open No. 2002-36524 was adopted and the thinning ratio at the boundary region was adjusted in accordance with the gradation without providing the overlapped region, it was recognized that the joint streak was not reduced sufficiently or the joint streak was made more conspicuous on the contrary. The reason the joint streak was not reduced sufficiently is that conventionally, the number of dots was adjusted only in specific regions adjacent to the boundary part despite the fact that the range in which the joint streak appears (thickness of the joint streak) varies depending on a variety of conditions, such as the kind of ink, printing medium, etc.